Modulation of Cosmic Ray
The flow rate of cosmic rays
incident on the Earth’s upper atmosphere is modulated by two processes: the
sun’s solar wind and the Earth’s magnetic field. The solar wind is an expanding magnetized plasma generated by
the sun, which has effect on the incoming particles, as well as some of the
particles with energies below about 1 GeV (Caballero-Lopez and Moraal, 2004). The amount of this wind is not constant due to
changes in the solar activities. Hence, the level of modulation varies in
autocorrelation with solar activities, (Ngobeni, 2006).
For earth’s magnetic field, the
cosmic rays deflect by the fact that the intensity of cosmic radiation is
dependent on latitude and longitude. The cosmic ray flux varies from eastern
and western directions due to polarity of earth’s geomagnetic field and
positive charge dominance in primary cosmic rays. This can be understood by the fact that
charged particles tend to move in the direction of the fields along and not
across them (Ngobeni, 2006). Cosmic Rays
entering the atmosphere is affected by the interplanetary magnetic field and
solar wind. This results in the modulation of their total flux and differential
energy spectrum as measured in the vicinity of periods of high solar activity.
This does not allow a direct application of the force field method to the study
of the atmosphere transport of cosmic rays (Gleeson et al, 1968).
However, despite
its limited theoretical application, the force field model provides a useful
way to parameterize the shape of cosmic ray differential energy spectrum. The modulation of cosmic rays varies with the
varying solar activity and is often described in terms of the so-called force
field model (Gleeson and Axford, 1968).
Natural Occurrence of Runaway Electron
In nature, runaway phenomena
occur in lightning path and a variety of astrophysical situations, studies of
which date at least back to 1925 (Wilson, 1925). Charge separation in a
thundercloud results in buildup of an electric field between the top and
bottom. One might expect that the breakdown potential for lightning in a
thundercloud is close to the typical potential measured in air, but
investigations have revealed a much lower breakdown potential (Gurevich, 1992;
Aleksandr et al, 2009).
This occurs due
to the above avalanche electron in which low-energy electrons are knocked into
runaway by high-energy electrons, with the original high-energy electrons
presumably generated by a cosmic ray or radioactive decay of an atmospheric
atom. Such high-energy electrons can have a mean free path of a few kilometers
over which they gain even more energy.
Runaways generated in lightning paths resulted
in a temporary nuclear monitoring site around nuclear reactors in Japan, where
they observed radiation bursts (now associated with runaways) during lightning
storms (Tsuchiya et al, 2007).
Runaway
is also observed in the outer radiation belt and at the Earth's magnetic bow
shock. In these situations, the accelerating fields come instead from interactions
between the Earth's magnetic field and the solar wind, such as reconnection of
magnetic field lines (Vasyliunas,1980) or from low-frequency radio waves (Zong,
2009).
Gurevich’s Runaway Breakdown Theory
The theory of lightning initiation is known as the “runaway breakdown
thoery”. It states that the energy inside a thundercloud, that force of
positive and negative particles, is too weak to generate a spark to initiate
lightning (SADF, 2009). Therefore, the thundercloud must be
struck by outside particles. These outside particles are burst of electrons
that carry very high energy. With this added energy, a spark can be generated
to initiate lightning. These outside particles do not come from the cloud above, or anywhere else on earth,
but from cosmic rays (Gurevich, 1992).
This theory
proposed by Aleksandra Gurevich of Lebedev Physical Institute in 1992 suggests
that lightning strokes are triggered by cosmic rays which ionize atoms,
releasing electrons that are accelerated by the electric fields, ionizing other
air molecules and making the air conductive by a runaway breakdown, then
causing a lightning stroke. (Gurevich, 1992; Shrope, 2004). In the atmosphere, electrons and ions with
charge, undergo an accelerating force from any electric field E present
(Alexander, 2011).
When a weak
electron field is applied to the atmosphere, the electron distribution develops
a drift, a slight distortion and at very high energies, a runaway electron
tail. The high energy tail extends to infinite momentum (or rather grows
indefinitely with time) (Kruskal and Bernstein, 1962). The waves that interact
with the runaways do not produce significant radial diffusion so that with
formed magnetic surfaces, it is unlikely that the radial loss of runaways
determines the steady state (Molvig and Tekula, 1977).
Runaway electrons are
thought to be accelerated by quasi-electrostatic fields in the middle
atmosphere following a positive cloud-to-ground (+CG) discharge (Bell et al., 1995), the cause
for the relativistic runaway electron avalanche being provided by MeV electrons
from a cosmic ray shower (McCarthy and
Parks, 1992).
Relativistic Runaway electron breakdown is an avalanche multiplication process
proposed to occur in moderate electric fields in gases. Electron breakdown
multiplication occurs whenever the electric field exceeds the threshold for
runaway relativistic electron breakdown but cannot begin without the presence
of the cause of relativistic electrons, requiring at least one energetic
particle to initiate the process. Under normal conditions, the Earth’s
atmosphere has many such energetic particles resulting partly from radioactive
decay but largely from cosmic rays (CRs) and the extensive air showers (EASs)
of secondary particles they produce (Nikolai, 2000).